Frequency converters are used, for example, to supply an AC current for driving electric motors. For this purpose, AC voltage of constant frequency of a multi-phase power supply system is converted by a rectifier to a DC voltage for charging a DC link capacitor. The DC voltage of this DC link is then converted by an inverter to an AC voltage of desired frequency and phase to suitably drive an electric motor. The frequency converter of an electric motor draws energy from the supply grid. This energy is ultimately supplied to the electric motor, where it is converted into kinetic energy.
To operate the frequency converter, a supply voltage of, for example, 24V is needed which is supplied to the control device for driving the semiconductor switches of the frequency converter through pulse-width modulation (PWM). Typically, this supply voltage is also taken from the DC link of the frequency converter by means of a switched-mode power supply.
Switched-mode power supplies are configured similarly to a frequency converter, and initially rectify the line voltage in order to charge a DC link. Using semiconductor switches, this DC voltage is converted to an AC voltage whose frequency is significantly higher (15-300 kHz) than that of the line voltage (50-60 Hz). It is this higher-frequency AC voltage that is fed to a transformer which provides at its output a suitable voltage that, after being rectified, is then available for operating the frequency converter.
The advantage of switched-mode power supplies is that the transformation of AC voltage occurs with much higher efficiency at higher frequencies. Unlike conventional power supplies, there is no need for a heavy iron core.
Switched-mode power supplies configured according to the known three-level phase shift topology (see, for example, G. Kácsor, P. {hacek over (S)}pánik, J. Dudrík, M. Luft, E. Szychta, Principles of Operation of Three-level Phase Shift controlled Converter, Electronics and Electrical Engineering, 2008. No. 2(82)) have four semiconductor switching elements for clocked application of a DC voltage obtained from an AC line voltage to a primary winding of a transformer.
However, like the frequency converter, the switched-mode power supply must be supplied with power to be able to drive the semiconductor switching elements thereof. Immediately after the frequency converter is switched on, the switched-mode power supply is unable to deliver a voltage because it cannot yet operate its semiconductor switches.
European Patent EP 0494327 B1 describes a start-up circuit which, after the switched-mode power supply is turned on, provides a supply voltage for the switched-mode power supply to start it up. The start-up circuit has a capacitive device and a resistive device interconnected to one another. The capacitive device is charged through the resistive device from a rectified AC line voltage until a control device can be powered from the voltage across the capacitive device.
German Patent Application DE 102008032316 A1 describes a start-up circuit for supplying power to control electronics of a frequency converter, where a capacitor is charged through a resistor from the DC link. After a certain time, a voltage sufficient to start up a PWM control device is then present across the capacitor. Once the switched-mode power supply has started to operate, the control device is powered via a self-supply winding of the switched-mode power supply. This document also addresses the problem that the start-up threshold of such circuits is not precisely defined because of manufacturing tolerances of conventional PWM control devices and the associated variance in the leakage current. In that document, this problem is remedied by using an additional switching element that ensures a defined switching hysteresis according to the applied input voltage.
When operating frequency converters with greatly varying power requirements, as is the case, for example, when using them in multi-axis machine tools, or in the case of a fluctuating line voltage, it is important that the switched-mode power supply be able to provide, under all operating conditions, a sufficient supply voltage for powering the frequency converter. In conventional frequency converters, this is not always guaranteed when high power is suddenly drawn from the common DC link of the frequency converter and the switched-mode power supply.